At present, the most frequently used method for in-situ measurement of stratospheric ozone concentrations is based on analyzing an electrochemical reaction of the ozone when it is passing through a special aqueous reaction solution in a measurement cell located in a balloon-borne sonde. The height range up to which useful measurements can be made using this method has an upper limit, which is primarily due to the fact that the pressure and temperature conditions of the measurement environment ultimately lead to a phase change of the aqueous reaction solution. Even a partial phase change frequently alters the aqueous reaction solution to such an extent that the measurement must be aborted. A fundamental physical limit is imposed by the triple point of the aqueous reaction solution. Measurements of ozone, a gas which is of great importance particularly in the stratosphere, are expensive and, thus, approaching the aforesaid limit is a goal to be optimized in each of these measurements.
The relevance of this problem is governed by the proportion of expensive sonde launches in which liquid phase conditions are left, and by the point at which, on average, this occurs within the ozone profile. Relevance analysis was performed based on 878 ozonesonde launches carried out during the last fourteen years from the Neumayer Station in the Antarctic, which is run by the Alfred Wegener Institute for Polar and Marine Research. The results obtained are as follows: In about 75 percent of launches, ambient conditions were reached which caused an at least partial phase change of the aqueous reaction solution, as indicated by the measurements of pressure and temperature within the sonde housing. In 22 percent of cases, the temperature fell below the freezing point. In the remaining 53 percent of cases, the temperature exceeded the boiling point at the prevailing ambient pressure. On average, this occurred at a height at which about 30 percent of the estimated total ozone column was still above the sonde. This results in corresponding uncertainties in the measurement results. A lower estimate for the number of ozonesondes needed worldwide is given by the number of data sets received by the World Ozone and Ultraviolet Radiation Data Centre (WOUDC) in Toronto. In the period between 2000 and 2005, the average number of ozone profiles was about 2100 per year. All these profiles were recorded using electrochemical sensors, about 80 percent of which were electrochemical concentration cells (ECC).
Publication I (Forschungszentrum Jülich [Research Center of Jülich, Germany], Institute of Chemistry and Dynamics of the Geosphere, Troposphere, JOSIE Project, ozonesondes, ECC sonde, on the Internet at the URL http://www.fz-juelich.de/icg/icg-ii/josie/ozone_sondes/ecc), as of Jan. 18, 2007) describes the construction of the SPC-6A sonde of Science Pump Corporation, Camden, N.J., USA, and of the ENSCI-IZ sonde of EN-SCI Corporation, Boulder, Colo., USA. The two sondes are nearly identical in construction and operation, and are based on a development made by Komhyr, W. D. (Ann Geophys., 25, 203-210, 1969) in the year 1969, and according to which the ECC is an electrochemical cell having an anodic chamber and a cathodic chamber. Both chambers are made of TEFLON (i.e., polytetrafluoroethylene (PTFE)) and contain platinum wire gauze electrodes which are immersed in aqueous potassium iodide reaction solutions of different concentrations. Potassium iodide is the white potassium salt of hydriodic acid, which dissolves very easily in water, cooling down significantly. The ion concentration required by this method results in special boiling and melting points for the aqueous reaction solution. The boiling and melting points are pressure-dependent, resulting in corresponding function curves. The function curves intersect at the triple point. The two chambers of the ECC are connected by an ionic membrane which allows the flow of current but prevents exchange of aqueous reaction solution between the two chambers. The ECC requires no external power supply, because it obtains its driving power from the difference in the concentrations of the aqueous reaction solutions in the two chambers (e.g., 0.06 mol/l=1% potassium iodide to about 8.0 mol/l=saturated). A sampling pump made of TEFLON which is non-reactive with ozone forces the ozone-containing ambient air through the cathodic chamber, which contains the less concentrated KI reaction solution, thereby producing free iodine (I2). The reaction equation is: 2KI+O3+H2O->2KOH+J2+O2. At the cathode, the I2 accepts two electrons and is thereby converted to I2−, which migrates to the anode in the chamber containing the high KI concentration, where it loses the two electrons and converts back to I2. The acceptance and release of electrons at the electrodes produces current flow in the circuit connecting the electrodes. This current flow is proportional to the ozone concentration in the surrounding air. However, the description of the operation of the sonde does not take into account the influences of the temperature and pressure prevailing in the surrounding air. The entire sonde is placed in a box made, for example, of polystyrene foam to prepare it for the balloon ascent.
Publication II (“Electrochemical Concentration Cell (ECC) ozonesonde pump efficiency measurements and tests on the sensitivity to ozone of buffered and unbuffered ECC sensor cathode solutions”, Journal of Geophysical Research, Vol. 107, No. D19, 4393, 2002) describes how measurement results can be corrupted by the power of the pump, which passes the surrounding air through the aqueous reaction solution, and by the amount of the phosphate buffer frequently added to the aqueous reaction solution. The investigations presented in the publication do not take into account any influence of the temperature and pressure of the surrounding air.
U.S. Pat. No. 3,681,228 (Electrochemical Concentration Cell for Gas Analysis) describes a practical ECC. This ECC has a cell housing having two parallel, closely adjacent bores extending therethrough along nearly the entire length thereof, forming the anodic and cathodic chambers. The two chambers are connected along their entire length by an ion-permeable membrane and are filled to about half their height with aqueous reaction solution of the same type, but of different concentrations. Both chambers contain electrodes made, for example, of platinum gauze and have sealed, outwardly extending terminals. An additional vertical bore is in permanent communication with the cathodic chamber and serves as a reservoir for aqueous reaction solution during non-stationary use of the sonde. All chambers are sealed at the top and have vents which are provided by thin tubes extending downwardly into the air space above the reaction solution and upwardly beyond the chambers. These vents enable pressure equalization during the passage of surrounding air through the ECC and during outgassing of the aqueous reaction solution as the pressure decreases with increasing height. The vents also serve to remove gas which emerges from the reaction solution during boiling under low-pressure conditions at great heights. All components are made of a material (e.g., TEFLON) which is non-reactive with the aqueous reaction solution and the gases to be determined The fact that reference is made to the possible boiling at great heights suggests that this phenomenon is at least recognized, but no consideration is given to the effects that this phenomenon and possibly also the freezing of the aqueous reaction solution may have on the measurement results.
However, during use of the sonde at great heights, the influences that the temperature and pressure of the surrounding air have on the measurement results, and which may lead to the freezing or boiling of the aqueous reaction solution, are well-known. This is evidenced by the fact that various temperature-buffering methods are used in commercially available sondes to extend their usability to greater heights. At the beginning of the measurement, the aqueous reaction solution is usually still at room temperature. The critical temperature at the triple point is about 0° Celsius. The temperatures in the stratosphere below the height of the triple point pressure are in the range between about −20° Celsius and −100° Celsius. In order to retard the cooling of the aqueous reaction solution and thereby approach the maximum measurement height in a more or less effective manner, measuring sonde manufacturers preferably use the method of thermally insulating the measurement cell with a thick shell of lightweight plastic. The thermal insulation is limited in thickness, mainly for weight reasons, and therefore can often not solve the problem alone, and especially not under winter conditions. As a result, ice crystals form prematurely before the triple point pressure is reached. A frequently used method to prevent this is to create a thermal bridge to a battery which is provided in the sonde to power the pump and which gives off waste heat. Attempts have also been made with additional, electrical heating elements adjacent to the measurement cell, or with heating elements which obtain heat from slow chemical reactions. Such methods have fundamental problems in terms of controllability, which cannot be solved without significantly increasing the costs and adding weight, which, in turn, results in a reduction in the height of ascent of the sonde. Accordingly, for reasons of space, weight and cost, the above-mentioned methods usually do not use any additional control systems for thermal coupling. Therefore, the risk of premature strong cooling of the measurement cell persists, especially at low temperatures in the stratosphere. On the other hand, under the conditions of higher stratospheric temperatures, there is an increasing potential for the aqueous reaction solution to overheat which, due the low ambient pressure in the stratosphere and the beginning evaporation of the aqueous reaction solution, also corrupts the measurement to such an extent that it must be aborted.